The role of positron emission tomography (PET) in diagnostics of gastroenteropancreatic neuroendocrine tumours (GEP NET).

PET is a successful modality to detect cancer and in recent years has demonstrated a great diagnostic value in large series of tumour types. PET combines high sensitivity and reasonable resolution, and offers the ability to perform whole body scans. 18F-deoxyglucose (FDG)-PET has also been used to diagnose tumours of neuroendocrine origin. Even if 18F-FDG has been successfully and widely employed in oncology, it has not demonstrated a significant uptake in well differentiated neuroendocrine tissues. Thus 18F-FDG is not a good tracer for neuroendocrine tumours, as FDG-PET imaging of number of GEP tumours revealed increased glucose metabolism only in less differentiated GEP tumours with high proliferative activity and in metastatising MTC associated with rapidly increasing CEA levels. In such a situation, additional 18F-FDG PET should be performed only if somatostatin receptor scintigraphy (alone or with 99mTc-DMSA) is negative. On the contrary, other positron emitter tracers seem to be more promising. 68Ga-DOTA-NOC (tetraazycyclododecanetetraacetic acid-[1-Nal3]-octreotide) has been used as a positron emitter tracer for the detection of NETs in preliminary studies. A serotonin precursor 5-hydroxytryptophan (5-HTP) labelled with 11C has shown an increased uptake in carcinoids. This uptake seems to be selective and some clinical evidence has demonstrated that it allows the detection of more lesions with PET than with CT or octreotide scintigraphy. Another radiopharmaceutical in the development for PET is 11C-L-DOPA, which seems to be useful in imaging endocrine pancreatic tumours.